Various sports, such as alpine, alpine/touring and telemark skiing, require boots that support the foot, ankle and, to varying degrees, the lower leg. All three of the above disciplines have these basic requirements. There are differences in the functional details and degree of support, but all three require that the foot, ankle and lower leg be supported in such a way that the range of motion is controlled within a specific range and that there is a specific resistance within the allowed range of motion that provides feedback to the skier and allows forces to be transmitted from the skier to the ski that would otherwise be impossible or difficult to apply.
As the need for more support developed, ski boot designs became stiffer and stiffer. Early ski boots were made from leather, then plastic coated leather, then designs eventually settled on the use of thermoplastic injection molded elastomers, such as thermoplastic polyurethane (TPU), polyamides, and blends such as Pebax®. Injection molded thermoplastics have been in use almost exclusively since the early 1970s.
There were some attempts to use fiberglass in the 1970s, notably the Raichle “Red Hot” ski boot. This boot would have been impossible to put on or take off due to the extremely rigid materials used. Raichle overcame the rigid nature of the materials by using a hinge along the sole that allowed the boot to split open from the top, toe to heel, and a leather upper that allowed forward flexing of the skier's ankle. Since the lower could not flex at all and maintained a fixed volume regardless of how it was closed, Raichle also had to provide for a way to fit the volume of the lower to various foot volumes and shapes. The following three problems, i.e., fixed volume lower, unnatural method of entry/exit and difficulty of mating a leather upper for forward flex to the rigid lower, prevented this boot design from achieving lasting success.
Recently, Lange/Rossignol attempted to use stiff composites to build competition ski boots for their sponsored World Cup skiers (see European Patent Publication No. EP1295540 B1, titled “Skiboot”). Lange/Rossingnol made several experimental ski boots using different combinations of composite materials. However, it appears those efforts have not yet resulted in any commercial ski boots incorporating the experimental concepts. The current inventor believes that one challenge the Lange/Rossignol designers may not have overcome is devising an entry/exit strategy that accommodates the extreme stiffness of the experimental boots due to the composites.
The only commercial use of composite materials in ski boot construction has been as inserts that are over-molded during the traditional injection molding of thermoplastics. For example, a small, shaped plate of composite material is prepared and then placed in a modified ski boot mold. Thermoplastic polyurethane, or other similar thermoplastic, is then injection molded around and partially over the insert to make it an integral part of the boot. This method uses the stiffness, strength and light weight of the composite material in areas of the boot where it can do the most good. However, it is not very economical as it requires very expensive molds, very expensive materials and very expensive preparation of the insert. Also, the weight and stiffness advantages of the composite materials are nearly erased by the heavy, rubbery thermoplastics that largely fail to efficiently transmit the forces they were designed to carry. Consequently these inserts are regarded primarily as cosmetic.